Any and every Irish government which attempts to erect customs posts at the land border with N. Ireland will fall, possibly never to be seen again. We fought a civil war over the issue and won't open that can of worms again. If the EU thinks that Theresa May is difficult to deal with, try dealing with an Irish government with it's back to the wall. IT WILL NOT HAPPEN. Even if there is wholesale smuggling across the border, some other means will have to be found to deal with it - random, roving customs patrols; trusted trader schemes; checks at Irish air and sea ports etc.

There is a long an honoured tradition in Ireland of laws formally enacted but totally ignored in practice dating back to the time when the British ruled and the Irish subverted wherever possible. This has been in rapid decline in recent years, but can always come back. Think of the black economy in Italy, Greece, Spain and many Eastern European states. It is characteristic of poorer, less rule bound societies or more divided societies with less consensus around the methods of government.

My German relations coming to Ireland used to be astounded at the very relaxed attitude to law enforcement and observance in Ireland when compared to "Vee haf rulz" Germany. That culture has changed in recent years, but you can force an Irish government to sign any law you like, it simply won't happen in practice. Ireland will veto anything it can in the EU until that situation changes. It is simply an existential issue for the Irish political establishment and many more besides.